1. Field of the Invention
The present invention concerns nuclear magnetic resonance tomography as applied in medicine for examination of patients. The present invention in particular concerns a movement-corrected multi-shot method for diffusion-weighted imaging in magnetic resonance tomography.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging method for over 15 years in medicine and biophysics. In this examination method, the subject is exposed to a strong, constant magnetic field (called the B0 field). The nuclear spins of the atoms in the subject, which were previously randomly oriented, thereby align. Radio-frequency energy can now excite these “ordered” nuclear spins to a specific oscillation. In MRT, this oscillation generates the actual measurement signal that is acquired by appropriate reception coils. By the use of non-homogeneous magnetic fields generated by gradient coils, the signals from the measurement subject can be spatially coded in all three spatial directions, generally designated as “spatial coding”.
In the evaluation of pathophysiological procedures, in particular in the human brain (for example given a stroke), a relatively new MRT technology has proven to be particularly effective: diffusion-weighted magnetic resonance tomography.
Diffusion is created by the thermal translation movement of molecules. It is a random process that is also designated as Brownian molecular movement. The distances traveled by the molecules considered in diffusion-weighted MRT measurements are very small; for example, unrestricted water molecules diffuse in a typical manner in every direction over a distance of approximately 20 μm in 100 ms or 60 μm in one second. These distances lie in the order of magnitude of individual cells, of human tissue.
By the use of markedly strong magnetic gradient fields (known as diffusion gradients) that, in this technique, are applied in a continuous manner or in pulses in addition to the spatially-coded gradient fields cited above, a collective diffusion movement of the respective molecules (in particular water) becomes noticeable as an attenuation of the magnetic resonance signal. Regions in which diffusion occurs therefore are more or less characterized as dark regions in the actual MRT image, dependent on the strength of the diffusion.
Diffusion-weighted MRT sequences typically are composed of three parts:
1. spin excitation (typically in the form of a slice-selective 90° RF pulse)
2. a diffusion preparation step and
3. an imaging readout module.
In its most general form, the diffusion preparation uses the typical Stejskal-Tanner technique, whereby a bipolar gradient pulse is switched with the two pulses separated by a 180° RF refocusing pulse. Newer sequences use a two-fold bipolar gradient pulse with an additional 180° RF refocusing pulse in order to reduce the influence of interfering eddy currents that would lead to image artifacts. In principle, after the diffusion preparation various imaging sequences can be used in the framework of the imaging readout module in order to generate diffusion-weighted images. One problem of the diffusion-weighted imaging is, however, the marked sensitivity to non-diffusion-like movement types such as: heart movement, breathing movement, etc. and the movements associated with these such as, for example, brain pulsation (movement of the brain in cerebral fluid). In particular in multi-shot sequences in MRT, such movements cause phase shifts in the nuclear magnetic resonance signal during the respective diffusion preparation, which leads to strong image artifacts. The use of diffusion imaging as a clinical examination method therefore has made possible for the first time the continuous development of faster measurement techniques such as, for example, echo planar imaging (EPI). EPI is a markedly faster measurement method in MRT. Given the use of single-shot echo planar imaging (SSEPI sequences), image artifacts that are created due to unpreventable movement types can be reduced or prevented. Movements as occur in conventional diffusion-weighted imaging sequences in effect can be “frozen” with SSEPI.
A disadvantage of SSEPI, however, is that, due to the low bandwidth per pixel in the phase coding direction, a strong B0 field dependency of the measurement signal exists. In regions with strong susceptibility gradients (such as, for example, in front temporal lobes or in the frontal lobes of the human brain) this leads to strong image artifacts. A general dependency of the image with regard to the eddy current-induced interferences of the basic magnetic field additionally exists. Since such eddy currents typically are induced by the diffusion preparation gradient pulses, these vary with the difference gradient direction and a quantity known as the b-value, which characterizes the diffusion. This leads to the image interference differing dependent on different preparation procedures and the image reconstruction then being impaired when diffusion-weighted images are combined in order to create parameter maps such as, for example, ADC (Apparent Division Coefficient) maps.
A further disadvantage of the SSEPI sequence is the very significant T2* dependency (T2* is the decay duration of the transverse magnetization due to local magnetic field inhomogeneities) o the very strong phase sensitivity, dependent on the type of the phase coding of an SSEPI sequence. Both result in strong image erasure artifacts and distortion artifacts, in particular in body imaging with the typically short T2 times of human tissue.
One possibility to prevent B0 sensitivities as occur, for example, in SSEPI is to use other single-shot sequences, for example RARE, HASTE or GRASE. RARE, HASTE and GRASE acquire the magnetic resonance signal in the form of a spin echo pulse train that is generated by emission of a number of radio-frequency refocusing pulses. The refocusing of the magnetization inverts the phase curve, making the sequence insensitive with regard to susceptibility artifacts. The T2 decay of the magnetization limits the signal readout time to approximately 300 ms, which in turn limits the maximum resolution that can be achieved. EPI as well as RARE, HASTE and GRASE can be improved with regard to the resolution by the use of parallel acquisition techniques (PAT).
In diffusion-weighted imaging, the readout of a number of spin echoes also has the severe disadvantage that movement-induced phase shifts of the magnetization occur during the diffusion preparation that negate the Carr-Purcell-Meiboom-Gill (CPMG) condition. This condition is fulfilled when the excitation pulses exhibit a 90° shift in comparison to the subsequent refocusing pulses, but is no longer fulfilled when a non-reversible phase change occurs between the RF excitation and the first refocusing pulse. A non-reversible phase change is a phase change that cannot be reversed by the refocusing pulse (echo-like) (for example a movement-induced phase change occurring in diffusion sequences). A reversible phase change is, for example, a phase evolution caused by resonance offset. Non-fulfillment of the CPMG condition and artifacts caused thereby are more likely to occur as fewer RF or refocusing pulses are used.
In order to prevent such artifacts, modified diffusion preparation techniques can be used that are based on stimulated echo sequences. However, these exhibit a two-fold variation of the signal-to-noise ratio (SNR). A single-shot spiral scan can also be used in order to acquire data in the framework of a diffusion-weighted imaging without artifacts (with regard to movement artifacts). Just like EPI, this technique does not use multiple RF refocusing pulses, which leads to a phase evolution due to off-resonant signals that severely impairs the image quality.
Multi-shot techniques represent a reasonable alternative in order to circumvent the disadvantages of single-shot techniques in diffusion-weighted imaging. Multi-shot techniques improve the image quality by increasing the spatial resolution; image artifacts that occur in the single-shot technique (due to T2 decay, T2* decay and off-resonance effects), and can exert a strong influence on the imaging due to the long readout time, can be prevented.
The use of multi-shot sequences in diffusion-weighted imaging presents the developer with new challenges. The simple combination of a standard multi-shot sequence with a preliminary diffusion preparation using diffusion gradients leads to a movement-induced phase shift from shot to shot that is manifested by extremely strong ghost artifacts, in particular when brain exposures are made. Initial studies of the human body in which simple single-shot sequences and SSFP sequences (steady-state free precession sequences) have been combined with diffusion gradients show no movement dependency whatsoever given b-values below 200 s/mm2. The b-value represents a value characterizing the diffusion-weighted measurement and is calculated according to a formula involving the condition of the diffusion gradients and of the gyromagnetic ratio of the considered magnetic resonant spin type. In the framework of standard examinations, given acute stroke such b-values amount to approximately 1000 s/mm2, such that under these conditions ECG-triggered SE sequences, namely ECG-triggered stimulated echo sequences (STEAM) present a strong influence of the brain movement in multi-shot diffusion-weighted images.
There are various approaches to counteract the movement sensitivity of multi-shot methods. A sequence-based approach is to acquire the signal in the framework of multiple small-angle excitations immediately at the temporal end of the diastole of a heart cycle, in the framework of a high-speed STEAM technique. This method can likewise be understood as a single-shot approach in which a single preparation of the magnetization is implemented for all excitations. This technique shows all the advantages of an EPI sequence without dependency of susceptibility changes. However, a limitation of the resolution exists. High-speed Steam likewise exhibiting the disadvantage of a relatively low signal-to-noise ratio.
An important development in the field of diffusion-weighted multi-shot imaging was the idea to measure a signal known as a navigator echo in addition to the conventional-image data after each spin excitation. The data of each navigator echo are used for phase correction of the corresponding image data, with the phase change of the signal that occurs during the diffusion preparation, and varies between the different excitations, being taken into account. These navigator echoes (in the form of a non-phase-coded reference scan) are in principle one-dimensional and can actually be used only for correction of general phase changes or regional phase changes in the read-out direction. The movement-induced phase change in the diffusion-weighted imaging is a two-dimensional function, such that the one-dimensional approach is not suitable for a complete correction. This method consequently is not suitable for a clinical implementation in the form of a routine application.
For this reason, present movement-corrected diffusion-weighted imaging sequences use two-dimensional navigator signals that are interleaved with the conventional acquisition sequence. Adjacent raw data lines are acquired in separate parts. However this leads to the scanning (sampling) not fulfilling the Nyquist condition and wrap-arounds occurring in the image domain. A direct, uncomplicated application of the two-dimensional movement correction (phase correction) is possible only when simplified assumptions are made, for example that of a rigid body motion. Such an assumption of rigid body motion is not valid for the brain deformation, in particular of the ventricle and of the brain stem.
A technique known as PROPELLER (Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction) is an exception, in which, after each shot, a set of parallel adjacent k-space lines (resembling a propeller blade) are acquired by means of a spin-echo pulse train. The direction of a blade is rotated after each spin excitation, such that the entire k-matrix is two-dimensionally scanned in a star shape. Since each blade contains the central region of the k-matrix, each shot is provided with 2D navigator information, which is why the sequence can also be designated as self-navigating. The Nyquist condition is also fulfilled for each blade, so a low-resolution image-based 2D phase correction is possible. Because in each shot the central k-space region is measured, the phase correction primarily concerns higher frequencies, which leads to a reduction of image artifacts. However, with PROPELLER the CPMG condition generally is not fulfilled, which leads to a signal modulation between adjacent spin echoes. The signal modulation concerns adjacent lines due to the T2 relaxation and due to movement-induced effects. A radial scan as is implemented with PROPELLER is also less efficient in comparison to standard scan sequences such as, for example, segmented EPI, since in comparison to other sequences a much larger number of shots is necessary for a given matrix size.
Furthermore, it has been proposed to use multi-shot spiral scans in the framework of self-navigated diffusion-weighted sequences. However, such developments in this field do not include the two-dimensional phase correction necessary for a robust clinical application.